Ultrasound has become popular among rheumatologists as the first-choice imaging investigation for the evaluation and monitoring of osteoarthritis (OA). Because of recent improvement in technology, ultrasound has the ability to demonstrate and assess the minimal structural abnormalities, which involve the pathophysiology and progression of OA, such as articular cartilage, synovial tissue, bony cortex, and other soft tissue. Nowadays, ultrasonography is a promising technique for assessing soft tissue abnormalities such as joint effusion, synovial hypertrophy, Baker cyst, and other structural changes including the decrease in cartilage thickness, meniscus bulging, and formation of osteophyte. Ultrasonography not only possesses diagnostic potential in knee OA but also reveals long-term predictability for disease progress as imaging biomarker. Ultrasonography has also been proven as a useful tool in guiding therapeutic interventions and monitoring treatment effectiveness. This review addresses the utility, reliability, and potential utilization of ultrasonography as an imaging technique in knee OA.